1. Field of the Invention
The invention is generally related to fiber reinforced metal matrix materials. Particularly, the invention is directed to a great improvement in the ductility of metal matrix materials.
2. Description of the Relevant Art
Ductility and energy absorption capacity are governing criteria for the selection of materials and structural systems in diverse applications including crashworthiness and impact resistance. In fiber reinforced composites with different matrices, frictional fiber pull-out offers the potential to absorb substantial energy. This potential source of energy absorption is, however, largely untapped because localized failure of matrix leads to localized deformation and rupture of fibers, which prevent activation of the frictional pullout process.
Shape-memory alloys will, after an apparent plastic deformation, return to their original shape when heated. The same class of materials, in a certain temperature range, can be strained up to approximately 10% and still return to their original shape when unloaded. These effects are called shape-memory and pseudoelasticity. Both effects depend on the occurrence of a specific type of phase change known as martensitic transformation. Pseudoleastic strains, which can be as large as 10%, are distributed within the material volume as are the relatively small elastic strains. Pseudoelastic strains nucleate at critical sites, including highly stressed sites, and then gradually spread along the length of the pseudoelastic fiber, eventually affecting the whole volume of the fiber (Shaw, J. A. and Kyriakldes, S., Material Characterization of NiTi Shape Memory Alloys: I Experiments, AMD-Vol. 200/MD-Vol. 57, Plastic and Fracture Instabilities in Materials, ASME, 1995, pp. 81-84). In conventional ductile metals such as steel, strains of the order of few percent, which are plastic strains, tend to localize and have little effect on the bulk volume of the material. Examples of shape-memory materials include nickel-titanium alloys, copper-based alloys such as Cu--Zn--Al and Cu--Al--Ni, and iron-based alloys.
Poisson's ratio is the ratio of transverse to longitudinal strains as the material is subjected to longitudinal stresses. The distributed nature of pseudoelasticity implies that the transverse strains associated with the Poisson's effect are also distributed.
Shape memory alloys have been used in a variety of composite materials. U.S. Pat. No. 5,614,305 to Paine et al. discloses hybridization of a brittle fiber reinforced polymer composite laminate with shape memory fibers which exhibit martensite phase transformation for the improvement of impact strength and resistance to delamination and perforation. These improvements are obtained through dissipation of strain energy in shape memory fibers as they undergo stress-induced martensite phase transformation. U.S. Pat. No. 5,508,116 to Barrett discloses a metal matrix composite exhibiting shape memory characteristics, which comprises shape memory alloy particles and metal particles consolidated to form a unitary mass. U.S. Pat. No. 5,611,874 to Zadno-Azizi et al. discloses a composite structure with shape memory cladding which exhibits shape memory characteristics and benefits from such physical characteristics of the body material as high conductivity, weldability, and solderability.
U.S. Pat. No. 5,614,305 to Paine et al. improves the impact strength, and delamination and perforation resistance of polymer matrix composites through martensite phase transformation in shape memory fibers at relatively small deformations. The associated improvements in ductility and energy absorption capacity are relatively small when compared with those obtained in this invention with metal matrices reinforced with shape memory fibers where the pull-out process of such fibers from the metal matrix dissipates substantial energy at large deformations. U.S. Pat. No. 5,614,305 to Paine et al. seeks to provide local improvements in the performance of polymer matrix composites, while this invention uses the full volume of metal matrices and shape memory fibers to provide global improvements in ductility and energy absorption capacity for applications such as crashworthiness and blast- or earthquake-resistant structures.